The present invention pertains to wireless communication technology and pertains in particular to the allocation of antenna patterns in a base station that is capable of transmitting in a specific direction using a plurality of antenna elements such as an array antenna.
In cellular wireless communications, the utilization of an array antenna is considered with the objective of improving antenna gain or reducing the interference that a certain terminal causes to the communications of another terminal. With an array antenna, signals from respective antennas are transmitted or received by attaching array weights cons sting of complex numbers to a plurality of antenna elements. As a result thereof, the antenna gain is enhanced in a specific direction with respect to the respective antennas and it is possible to supply a directional pattern. This signal processing technology is called <<beam forming>>. As for the granting of array weights, it has gradually become common for it to be controlled by means of digital signal processing and array weighting is freely controlled with specific timing, so it is possible to modify the directivity of the antennas. By utilizing this technology, the antenna gain is modified adaptively with respect to the movement of users inside the cell (or sectors) covered by a base station, so adaptive array processing devised to supply a directivity pattern that is at all times optimal for the individual users also becomes possible.
In next-generation cellular wireless communication, OFDM (Orthogonal Frequency Division Multiplexing) is used as the modulation method. In OFDM, a signal is transmitted by decomposing it into mutually orthogonal frequency components by means of signal processing using FFT (Fast Fourier Transform). In wireless communication utilizing OFDM, it is possible, by providing computation in which the aforementioned array weights are added for each decomposed frequency tone, to confer a different directivity pattern for each frequency. Examples of such technology are mentioned in IEEE C802.20-05-59r1 http://ieee802.org/20/ QFDD Technology Overview Presentation (Nov. 15, 2005), Slide 68. In Slide 68, processing that changes the array weights for each user in OFDMA (Orthogonal Frequency Division Multiple Access) is disclosed.
When determining the array weights of the downlink, the communication line from a base station to a terminal, downlink information is estimated from uplink information and array weights are controlled on the basis of the result of the estimation. However, in an FDD (Frequency Division Duplex) system, it is difficult to estimate downlink information from uplink information and the implementation of adaptive array processing ensuring a good C/I (channel to interference) ratio at all times by adaptively changing the array weights is difficult. As a result, the communication environment is ensured by carrying out signal transmission or reception by changing a fixed directivity pattern temporally or frequency-wise and adapting to the timing or the frequency at which the beam is oriented in the user's own direction.
In e.g. JP-A-2007-243258, there is disclosed antenna pattern allocation technology in which, in a system constituted by a plurality of base stations, either of two orthogonal directivity patterns is appropriately allocated to the time axis or the frequency axis with respect to each base station. According to JP-A-2007-243258, signal transmission based on frequency or time for which interference from adjacent base stations is avoided becomes possible and packet scheduling in which strong interference from adjacent base stations is avoided, due to the combination with a scheduler, becomes possible.